The towers don't exibit this. They're pretty much the same at low-levels as at higher levels, and they don't change after "break-in".
I never noticed any difference with my towers.
As a side note, I never finished the cabinets on mine and when we moved a couple years ago they got stored in the garage due to lack of room. This last summer a friend and I were talking about the towers so I pulled them out into the garage and ran a CDP thru my old Carver amp into them....then proceeded to blow him away!!!! Not only was he impressed by the volume level , but he said they were the best speakers he had ever heard. And these were set up in a garage